Tape lettering systems employing a dry lettering printing process that mechanically transfers an impression of a character on a rotatable type disc from a dry film ribbon to an image carrying tape by means of an impact means or pressure printing force are well known in the prior art, and are shown and described in U.S. Pat. Nos. 2,834,507; 4,243,333; and 4,402,619. An automated tape lettering machine employing this process is shown and described in U.S. Pat. No. 4,462,708. While each of these prior art machines is capable of generating high quality printing and lettering results, there is a need for a high speed tape printing apparatus capable of generating high quality characters without the limitations imposed by using an impact or pressure lettering device.
Thermal transfer printing devices also exist in which an image of a desired character is formed on a strip of image carrying tape by transferring ink or other color from a color carrying ribbon to the tape as a result of the localized application of heat and a small amount of pressure. A typical thermal transfer device of this type is described in U.S. Pat. No. 4,666,319. Another thermal transfer device presently available employs a thermal print head for transferring images from a strip of ribbon to a strip of tape and has a cooperating tape-ribbon cartridge for providing a supply of tape and ribbon to the device. While such devices are useful for printing smaller point size characters represented in a dot-matrix array font format, the control systems required by such devices are not capable of handling the precision and accuracy required by a high speed tape printing apparatus capable of generating high quality characters, particularly characters of larger point sizes.
One of the difficulties in implementing a control system for a high speed, high quality thermal transfer tape printing apparatus is controlling the temperature of the individual pixel heating elements in the thermal printhead. Prior art systems for controlling the temperature of the pixel heating elements in a thermal printheads have only been applied to smaller print heads used in more conventional printing applications (e.g. 5.times.7, 24.times.1 dot matrix printheads), and have not been applied to a large thermal printhead (e.g., 256.times.1) with relatively small pixel heating elements of the type used in the thermal transfer tape printing apparatus contemplated by the present invention.
In U.S. Pat. No. 4,376,942, a preheat power is applied to the individual pixel heating elements only on the current state or value of the pixel. If the current pixel state is black or "on", then no preheat power is applied. If, on the other hand, the current pixel state is white or "off", a constant preheat power (of a reduced level) is applied for the entire duration of that pixel print time. While this method has the advantage of keeping the temperature of the individual pixel heating elements close to the threshold print temperature needed to effect a thermal transfer, thus minimizing the time needed for that individual pixel heating element to reach the threshold print temperature, it requires a large amount of power for a printhead with a large number of pixels. This results from the fact that during the operation of the device of the above '942 patent, each pixel heating element is constantly energized, or consumes some type of current (either a preheat or a print current), during the entire time of each print cycle. Further, the '942 device fails to compensate either for consecutive occurrences of the same pixel state, (i.e., three consecutive "print" cycles), or for long term application of the preheat power (i.e., several consecutive "no print" cycles). Further, the circuitry is quite complicated because two preheat voltage levels are required. All of these conditions may lead to an overheating of the entire printhead or of the individual pixel heating elements and result in a smearing of a pixel image on the tape or the creation of ghost or phantom images where no images are intended or desired.
In U.S. Pat. Nos. 4,415,907 and 4,560,993, hardware circuitry is added to the printhead circuitry to base the preheat power on a combination of the previous pixel state or value and the current pixel state or value. In the system of U.S. Pat. No. 4,560,993, the objective is to minimize the power consumption of the printhead by warming up or preheating the printhead with a preheat pulse only when (i) the current pixel state is to "print" and (ii) the previous pixel state was to "not print". Thus, in U.S. Pat. No. 4,560,993, energy is conserved by removing the pixel preheat if the previous pixel state was to "print" (or was black) and the current pixel state is also to "print" (or is black) and by removing the pixel preheat whenever the current pixel state is to "not print" (or is white). While combining the previous pixel state with the past pixel state increases the ability to reduce power consumption and the overheating of the printhead or individual pixels during continuous operation of the system, the systems taught by these prior art patents require extensive additional circuitry to implement, especially for larger size thermal printheads.
Each of these systems also suffers from the disadvantage of not being able to precisely maintain the temperature of the individual pixel heating elements close to, but just below, their threshold thermal transfer temperature. Maintaining the pixel heating elements just below this threshold level decreases both the amount of power and the time required to activate an individual pixel heating element. In addition, because the ribbon used in high quality thermal transfer tape printing apparatuses is usually some type of plastic based ribbon, the pixel images transferred from the ribbon to the tape are uniformly defined and can not smear or smudge into one another. Consequently, unlike the thermal printers and pixel preheat systems of the prior art, the uniformity of the pixel images created by the thermal transfer apparatus of the present invention are not separated by a blank space, but must be exactly abutting the adjacent column of pixel images. This requires an even more precise control of the temperatures of the pixel heating elements to enable the pixel heating elements to be turned on and turned off quickly and accurately in order to achieve high quality, high speed lettering results when using a thermal transfer tape printing apparatus of the type contemplated by the present invention.
Although the current pixel preheat systems for thermal transfer devices may be satisfactory for various uses and applications, they are limited in their application to large thermal printheads and do not provide sufficient control over the pixel heating element temperature to enable high quality characters to be printed on the tape at a high speed. Accordingly, there is a continuing need for improvements in the control systems associated with tape lettering printing apparatus, and, in particular, with an efficient pixel preheat system for such thermal transfer devices to allow for a more precise control of the temperature of the individual pixel heating elements in the printhead.